1. Field of the Invention
The present invention generally relates to the field of wireless communication systems. More particularly, the present invention relates to a novel and improved apparatus and method for calibrating the crystal oscillator in a receiver to derive more accurate timing.
2. Related Art
In wireless communication systems, such as code division multiple access (xe2x80x9cCDMAxe2x80x9d) systems, the transmitter and receiver are time synchronized for use in signal demodulation and decoding. In CDMA systems each user uniquely encodes its message signal into a transmission signal in order to separate the signal from those of other users. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the message.
Transmitter and receiver time synchronization can be achieved by transmitting a pilot signal from a base station to a mobile unit. The mobile unit can use the pilot signal to correct a local oscillator which the mobile unit uses as a timing reference. For example, in IS-95 and cdma2000 systems, by tracking the pilot signal, the mobile unit may obtain an accurate timing reference from the base station timing, which is derived from GPS.
The local oscillator used as a timing reference by the mobile unit is typically a crystal oscillator. The crystal oscillator may be a voltage controlled oscillator (xe2x80x9cVCOxe2x80x9d), for example, which uses a reference or tuning voltage to control, i.e. to change, the frequency of the oscillator. Other types of frequency control are also possible. For example, an oscillator could be a current controlled oscillator.
The oscillation frequency of a crystal oscillator may be affected by changes in the ambient temperature. A temperature compensated oscillator is designed to reduce or compensate for the change of oscillation frequency due to changes in temperature.
The oscillation frequency of the crystal oscillator is usually specified as a nominal frequency within a range of tolerance. For example, an oscillator may be specified as a 19.68 MHz (Mega-Hertz or million cycles per second) frequency oscillator rated at +/xe2x88x925 parts per million (xe2x80x9cppmxe2x80x9d) error. Then, for example, if the 19.68 MHz oscillator is used to synthesize an 800 MHz carrier for application to an RF mixer, the synthesized carrier frequency applied to the RF mixer may be expected to be accurate to within 4,000 Hz error (5 parts per millionxc3x97800 million Hz or 5xc3x97800 Hz). The frequency error may be corrected or compensated for by using, for example, an automatic frequency control loop composed of a frequency error detector, a loop filter, and a voltage controlled oscillator.
The frequency error detector, which is often referred to as a discriminator, computes a measure of the difference between the received carrier frequency and the synthesized carrier frequency, referred to as an xe2x80x9cerror measurexe2x80x9d. This error measure is filtered to produce a xe2x80x9cdigitalxe2x80x9d control signal that is converted into an analog tuning signal that is fed to the voltage controlled oscillator. Tuning the voltage controlled oscillator modifies the frequency of the synthesized carrier. In matching the received carrier frequency with the synthesized carrier frequency, this closed loop feedback corrects the timing of the local oscillator.
The frequency error of an oscillator may be affected by several factors. These factors may include (i) temperature, as noted above, (ii) aging of the crystal and other components, (iii) differences in operating voltages within and between mobile units, and (iv) differences in components from one mobile unit to another.
The multitude of different factors and their variability over time cause considerable difficulty in calibrating the digital control signal that is applied to correct the timing of the local oscillator. For this reason, calibration is typically not performed in current wireless mobile systems. Instead, frequency tracking using the feedback of an automatic frequency control loop is relied upon to push the digital control signal in the correct direction. In other words, the digital control signal has the correct arithmetic sign, positive or negative, but the magnitude of the digital control signal is not sufficiently exact for certain purposes in which it is desirable to translate a known, or predetermined, frequency error directly into a digital control signal to correct the timing of the local oscillator.
FIG. 1 illustrates a previous approach using frequency tracking in an automatic frequency control loop in a CDMA wireless communication system. Frequency tracking system 100 shown in FIG. 1 might, for example, constitute part of a receiver in a CDMA mobile unit. Frequency tracking system 100 may communicate, for example, via radio frequency (xe2x80x9cRFxe2x80x9d) signal propagation between a base station transmit antenna (not shown) and receive antenna 102 connected to RF front end 104. RF front end 104 typically uses frequency synthesizers, which match the frequency of the RF carrier, to convert the RF signal to a baseband frequency signal, i.e. the encoded message signal before it was modulated onto the RF carrier for transmission which is more concisely referred to as a xe2x80x9cbaseband signalxe2x80x9d.
The frequency synthesizers used by RF front end 104 receive timing reference 103 from local oscillator 106, which is a voltage controlled oscillator (xe2x80x9cVCOxe2x80x9d) in the present example. As seen in FIG. 1, the digital baseband signal has an in-phase component, referred to as I component 105, denoted xe2x80x9cIxe2x80x9d in FIG. 1, and a quadrature component, referred to as Q component 107, denoted xe2x80x9cQxe2x80x9d in FIG. 1.
Continuing with FIG. 1, I component 105 and Q component 107 of the digital baseband signal are fed as an input signal to frequency error discriminator 110. Pilot demodulation module 112 demodulates the input signal as a sequence of symbols, also referred to as a xe2x80x9csequence of pilot symbolsxe2x80x9d. Each pilot symbol has an I component 113 and a Q component 115 so that it can be represented as a vector in a 2-dimensional IQ plane, where the I component lies on the horizontal axis, and the Q component lies on the vertical axis. A frequency error, i.e. a mismatch between the carrier frequency synthesized to demodulate the incoming signal in RF front end 104 and the incoming carrier frequency, in local oscillator 106 causes the received sequence of baseband pilot symbols to rotate around the 2-dimensional IQ plane.
Each pilot symbol composed of I and Q components 113 and 115 is fed to unit delay elements 114 and 116, respectively, and also to phase rotation measure module 118. Unit delay elements 114 and 116 make previous pilot symbol composed of I and Q components 117 and 119, respectively, available to phase rotation measure module 118 at the same time as current pilot symbol composed of I and Q components 113 and 115, so that phase rotation measure module 118 can compute the phase rotation between successive pilot symbols. Each pilot symbol is represented as a vector in a 2-dimensional IQ plane, where the I component is mapped to the horizontal axis, and the Q component is mapped to the vertical axis.
Any 2-dimensional vector (x,y) can be represented in polar coordinates as (r, xcex8), where r={square root over (x2+y2)} and   θ  =                    tan                  -          1                    ⁡              (                  y          x                )              .  
If, for example, the current pilot symbol composed of I component 113 and Q component 115 is represented in polar coordinates as (r1, xcex81), and the previous pilot symbol composed of I component 117 and Q component 119 is represented in polar coordinates as (r0, xcex80), then phase rotation measure module 118 outputs error measure 121 represented as the phase difference (xcex81xe2x88x92xcex80) between successive pilot symbols. Thus, error measure 121 is directly proportional to the frequency error in local oscillator 106. Phase rotation measure module 118 outputs error measure 121 to gain xcex1 filter 122.
Continuing with FIG. 1, error measure 121, which may be a summed or averaged error measure, is fed to gain xcex1 filter 122. Gain xcex1 filter 122 provides a filtered error measure in the form of control bits 123 to control register 124. Control register 124 provides storage for control bits 123, which are output as control bits 125 to digital to analog converter 126. Digital to analog converter 126 converts control bits 125, which digitally represent a control value, into an analog quantity, such as a voltage whose quantity is directly proportional to the control value. RC filter network 128 filters out any residual fluctuations in the output voltage of digital to analog converter 126 to provide analog tuning voltage 129, i.e. control voltage, to local oscillator 106. Analog tuning voltage 129 changes the oscillation frequency of local oscillator 106, which changes timing reference 103, which changes the frequency of the frequency synthesizers used by RF front end 104. Thus, FIG. 1 shows one example of a previous approach to frequency tracking for a receiver in a CDMA wireless communication system.
In the example described in FIG. 1, an inexact xe2x80x9ccalibrationxe2x80x9d of the digital control signal is provided, for example, by supplying an empirical value for control bits 125 and allowing the frequency tracking system to run until stable. A more exact technique is needed for those instances in which it is desirable to translate a predetermined frequency error directly into a digital control signal to correct the timing of the local oscillator, i.e. where accurate calibration for adjustment of the digital control signal is required.
Accurate calibration is helpful, for example, in pilot searching, where an automatic frequency control loop cannot be run to correct the frequency error in the local oscillator because the pilot signal has not been acquired yet. When the mobile unit is turned on, the mobile unit must first xe2x80x9csearchxe2x80x9d for the pilot signal. Typically, pilot searching is done by selecting an initial frequency for the local oscillator, and then performing a search for the pilot over a PN code space, i.e. by varying the phase of one or more PN codes. If the pilot is not found, the local oscillator frequency is adjusted upward or downward, and the pilot search is performed again over the whole PN code space. This process is repeated until the pilot signal is found. Without accurate calibration, frequency adjustments are not accurately controllable, reducing the efficiency of the frequency search.
Efficient and accurate calibration is also important in xe2x80x9cquick pagingxe2x80x9d. Quick paging works by placing the mobile unit in a reduced power consumption or xe2x80x9cidlexe2x80x9d mode. At regular intervals, the mobile unit xe2x80x9cwakes upxe2x80x9d and checks to see if it has any incoming calls. If the local oscillator is not correctly calibrated, the amount of xe2x80x9cwake upxe2x80x9d time required to check for incoming calls is too long, and thus, the power savings of idle mode can be substantially compromised.
Thus, there is a need in the art for more efficient and accurate techniques for calibration of local oscillator frequency in wireless communication systems. There is also need in the art for automatically calibrating, in the mobile unit of a wireless communication system, the control input to the local oscillator that is required to correct for the frequency error of the local oscillator. Moreover, there is a need in the art for automatically calibrating, in the mobile unit of a wireless communication system, the control input to the local oscillator that is required to correct for a predetermined frequency error.
According to the present invention, a receiver comprising a digital rotator in combination with a frequency error discriminator in a digital automatic frequency control loop is used to arrive at accurate digital values used to calibrate a local oscillation frequency.
In one aspect of the invention, a frequency error in the oscillation frequency of a local frequency generation loop causes a change in the baseband input signal frequency during demodulation of the input signal. The change in the baseband input signal frequency related to the frequency error in the local frequency generation loop can be detected, for example, as a phase rotation by the frequency error discriminator. By using the digital automatic frequency control loop, including the digital rotator and the frequency error discriminator, the frequency error introduced by the local frequency generation loop can be determined with a high degree of accuracy.
For example, the frequency error can be determined when the digital automatic frequency control loop operation becomes stable, i.e. the frequency error is no longer substantially changing. The frequency error and corresponding control bits are entered into a calibration table, for example, in a mobile unit""s memory. The calibration table may be used, for example, to adjust the local oscillation frequency for temperature changes, pilot frequency searching, and quick paging.